A conventional method for decoding a digital signal is described. The digital signal is a pulse-width modulated signal. The signal can assume two states, a high signal level and low signal level. In pulse-width modulation, a specific time, i.e., the total pulse width, is provided for each bit to be transmitted. During the total pulse width, the signal assumes first the low and then the high signal level. The duration of the high signal level includes either one-third or two-thirds of the total pulse width. The first case corresponds to a coded binary zero, the second case to a 1. Decoding of this bit is accomplished by measuring the signal level at approximately half the total pulse width. The decoder is equipped with an oscillator to measure reliably the middle of the total pulse width.
The need to equip the decoder with the oscillator makes the decoder more expensive. If longer bit streams are to be decoded, on the one hand, the oscillator in the decoder must be accurate. On the other hand, the total pulse widths of the individual bits must also be highly reproducible. Such requirement makes necessary the use of highly accurate and exactly tuned oscillators in both the decoder and the coder.
Unpublished German Patent Application No. 1 96 162 93.9 describes a bus system for a transmission of messages between a control device and a peripheral unit, such that the control device sends high-priority messages and low-priority messages to the peripheral unit. High-priority messages have a greater amplitude than low-priority messages. The messages consist of digital signals--a binary 0 corresponds to a low signal level, and a binary 1 to a high signal level.